1. Field of the Invention
The present invention relates to a method for preventing loosening of a bolt fastened to a metallic member coated with a thick anticorrosive paint film.
2. Description of the Prior Art
As a support member (commonly called "bracket") for fixedly securing an engine to a vehicle body of a motorcar, a member made of cold-rolled sheet steel, hot-rolled sheet steel or cast iron has been used. In general, such support members are subjected to anti-corrosion treatment, and members made of pressed sheet steel are subjected to plating treatment or electrodeposition painting, while members made of cast iron are subjected to dip coating.
The reason why members made of cast iron are subjected to dip painting is because their cast surfaces are not flat, and because the coated film is thick (a minimum film thickness being 30.about.60 .mu.m), after a bolt has been fastened the coated film positioned at the bolt seat surface portion would be subject to plastic deformation into a thin film as time elapses, and loosening of the bolt would occur. Therefore, in the prior art, in order to prevent such loosening of a bolt, a countermeasure was taken through the method of (1) performing masking of the bolt seat surface portion for dip painting, (2) forming a bolt seat surface portion by machining the painted member, or the like. However, if the base skin of the bolt seat surface portion is exposed through such methods, there occurs a disadvantage that rusting of the exposed surface will readily occur and the appearance is deteriorated. In addition, if a support member made of pressed sheet steel should be subject to dip painting and then a bolt seat surface portion should be formed thereon through the above-described procedure for the reason that the process of dip painting is more economical than a treatment process such as plating, electrodeposition painting or the like, rusting will occur on the exposed base skin of the bolt seat surface portion, resulting in lowering of the mechanical strength of the relatively thin pressed sheet steel. Accordingly, in the prior art, a support member made of pressed sheet steel could not be subjected to dip painting but it had to rely upon the economically disadvantageous plating method or electrodeposition painting method. The present invention has been developed as a result of the above-mentioned technical background.